Inspection and review systems play an integral role in the semiconductor manufacturing industry. The primary role of inspection systems is to optimize the quality of manufacturing systems, which will minimize the number of semiconductor devices produced with defects thereby increasing yield. The performance requirements of inspection systems continue to increase as the size of semiconductor devices continues to decrease. For instance, the resolution and sensitivity requirements of inspection systems are becoming more stringent as they need to detect smaller sized defects in accordance with the smaller structures present on advanced semiconductor wafers. Another important consideration is the repeatability, or matching, of these inspection systems across multiple units, that is, it is important that one system sees exactly the same defects as another system on the same sample. These inspection systems can be used to inspect the semiconductor wafer at various stages of its fabrication and inspect lithography reticles also used in the fabrication processes.
The quality of light used for illumination with inspection systems is one factor affecting resolution, sensitivity, and matching. FIG. 1 shows a common illuminator 100 for an arc lamp source used in inspection systems. Ideally an illuminator produces a spatially uniform beam of light covering the full field of view as defined by the field stop of the inspection system at the sample under investigation. In addition, it should provide uniform angular illumination up to the full numerical aperture (NA) of the inspection system as defined by the entrance pupil. Any deviation from the uniform nature of the spatial and angular illumination will reduce the effectiveness of the inspection system through either less sensitivity or inability to match multiple inspection systems on the same specimen.
Illuminator 100 includes an illumination source 102, a rectangular lightpipe 104, optical lens 106, and an entrance pupil 108. FIG. 1 also illustrates a perspective view of rectangular lightpipe 104. Illumination source 102 causes light from source 110, which is located at one focal location of ellipsoidal reflector 114, to be collected at location 112, which is the second focal point for ellipsoidal reflector 114. Light at the exit of light pipe 104 will be conjugate to the field stop of the inspection system. Light pipe 104 is intended to make the light spatially uniform at the exit of light pipe 104. In one embodiment, illuminator 100 can use a microscopic-type illumination source for illumination on a brightfield semiconductor wafer inspection tool. Illuminator 100 illuminates an inspection field of view on a sample, such as a semiconductor wafer, by injecting light into an inspection system through entrance pupil 108.
Illumination source 102 typically includes an arc lamp 116 of Mercury (Hg) or Mercury Xenon (HgXe), which generates light in all directions. To effectively direct light towards second focal point 112 and lightpipe 104, an ellipsoidal reflector 114 is used to collect as much light as possible and reflect the light from arc lamp 116. Unfortunately, various factors cause light emanating from illumination source 102 to be non-uniform in nature when the light reaches entrance pupil 108. Non-uniform illumination light at entrance pupil 108 causes deterioration of inspection system resolution since the resolution of an inspection tool is a function of the Fourier transform of the light distribution at entrance pupil 108 along with the phase of the lenses aberrations. So a non-uniform pupil 108, especially one with an illumination falloff towards the edges of pupil 108, will reduce resolution. Another factor affecting sensitivity is the amount of light available throughout entrance pupil 108. This can be important when applying illumination aperture techniques which use light blocks in the pupil to provide only certain illumination angles at the wafer. These techniques can increase sensitivity of certain classes of defects on wafer samples. If there is not much light near the edge of the pupil, then illumination apertures which use light from the edge of the pupil will be less effective. Generally, non-uniform light causes a sample to be unevenly illuminated and thereby results in inspection signals having non-uniform sensitivity and will not match between inspection systems.
One of the factors affecting the uniformity of the illumination light is the inherent quality of the plasma within arc lamps. The plasma causes the light that leaves the lamps to have different intensities at different angles. Another factor is the non-uniformities of the reflective surface of ellipsoidal reflector 114 and any aberrations the surface might have from the ideal ellipsoidal surface. Yet another major factor is the large range of incident angles (from very low to very high) through which light from arc lamp 116 reflect off of ellipsoidal reflector 114. The reflectivity of the coatings on the ellipsoidal reflector will change as a function of incidence angle.
The light can be non-uniform in respect to spatial distribution and angular distribution. FIG. 1(a) illustrates a computer simulation of the distribution of light from illumination source 102 at a plane located at second focal point 112. This location also happens to be at the point before light enters lightpipe 104. FIG. 1(a) shows that the light is spatially non-uniform since it varies with respect to the radial distance from the center of the circle of light.
Another factor causing non-uniform light distribution at entrance pupil 108 is that the arc lamp 116 generates light between an anode and a cathode. These very anodes, cathodes, and the wires that connect to these respective components block portions of the light generated from arc lamp 116. Inset FIG. 1(b) illustrates a computer simulation of the distribution of light from illumination source 102 at the plane 120. FIG. 1(b) shows that the light intensity varies with respect to the radial distance from the center of the circle of light. More notably however, FIG. 1(b) shows the shadow created by the anode, cathode, and wires that are connected each respective component.
Rectangular lightpipe 104 is used to compensate for the non-uniform spatial nature of the light from illumination source 102. Lightpipes scramble the light from the arc lamp 116 as the light bounces through lightpipe 104 as it travels through its length. Specifically, rectangular shaped lightpipes are used to scramble the spatial distribution of the light so that light is uniformly distributed in the plane of the field stop 105. FIG. 1(c) illustrates a computer simulation of the distribution of light from illumination source 102 after the light has been spatially distributed by passing through lightpipe 104 at field stop conjugate 105. As shown in FIG. 1(c), the light is substantially more evenly distributed in the spatial respect when compared to FIG. 1(a). After passing through lightpipe 104, the light is directed by optical lenses 106 towards entrance pupil 108. In one embodiment, lightpipe 104 has a 4.3 mm×1.72 mm rectangular outline while having a pipe length of approximately 150 mm.
Even though the non-uniform nature of the light from illumination source 102 is reduced with respect to spatial distribution at the field stop 105, the light remains non-uniform with respect to angular distribution, which is represented by the spatial distribution of light at the entrance pupil 108 after imaging by lens 106. The location of lens 106 is one focal length of lens 106 away from field stop conjugate 105 and the pupil is formed at about 1 focal length further down the optical axis from lens 106. Used in this manner, lens 106 changes the angular distribution of light at field stop conjugate 105 to spatial light distribution at entrance pupil 108. Thus, any non-uniformity of the angular distribution of light leaving field stop 105 will result in a non-uniform spatial distribution of light at entrance pupil 108. Rectangular lightpipe 104 does not re-distribute the angles at which light travels because the angles at which the light bounces off the internal surfaces of the rectangular lightpipe 104 are preserved. FIG. 1(d) illustrates a computer simulation of the distribution of light from illumination source 102 at entrance pupil 108. Unfortunately, it can be seen in FIG. 1(d) that the light at entrance pupil 108 is highly spatially non-uniform and this will result in non-uniform illumination angles at the specimen. The distribution of light at the pupil can adversely affect the system's resolution and its sensitivity to defect capture. In addition, the matching of one system to another system is dependent upon the same non-uniform illumination being achieved in each system's entrance pupil. Since this non-uniformity is a result of many factors already mentioned, it is difficult to achieve this requirement. In particular, the wires going to the arc lamp produce obscurations depending upon their shape and routing that are multiply distributed throughout entrance pupil 108 through the kaleidoscope effect from lightpipe 104.
Other types of illuminators use lens-type condensers instead of ellipsoidal mirrors to collect and direct light in into a lightpipe. These condensers also suffer from angular non-uniformities, which result in entrance pupil non-uniformities due to lens aberrations, anti-reflection differences as a function of incidence angle and the obscurations from cathode and anode and their connection wiring.
One attempt others have used to smooth out the light at entrance pupil 108 to increase system resolution and sensitivity involves using a diffuser. Generally, a diffuser is a rotating grounded glass plate or phase glass plate capable of reducing the artifacts from the arc lamp and its structures (anode, cathode and wires). However, it is difficult to achieve a uniform pupil with a diffuser as it tends to produce a Guassian distribution of light leaving it. Also, diffusers are generally not very light efficient, need to be rotated at high velocity (e.g., 10,000 to 20,000 rpm), and can be expensive.
Other illumination sources for wafer inspection and review systems include a laser source. The laser usually has a Gaussian distribution of light leaving it and methods of producing an angular and spatial uniform illumination at the specimen include both light pipes and diffusers of various types. In all cases, these suffer from the inability to remove the basic Gaussian distribution of the laser at either the entrance pupil or field stop.
In light of the foregoing, there are continuing efforts to provide improved techniques for distributing light uniformly across an entrance pupil of an inspection system while maintaining the uniform distribution across the field stop.